AD8627/AD8626/AD8625

Rev. B | Page 15 of 20

MINIMIZING INPUT CURRENT

The AD862x is guaranteed to 1 pA max input current with a

±13 V supply voltage at room temperature. Careful attention to

how the amplifier is used will maintain or possibly better this

performance. The amplifier’s operating temperature should be

kept as low as possible. Like other JFET input amplifiers, the

AD862x’s input current doubles for every 10°C rise in junction

temperature, as illustrated in Figure 8. On-chip power dissipation

raises the device operating temperature, causing an increase in

input current. Reducing supply voltage to cut power dissipation

reduces the AD862x’s input current. Heavy output loads can

also increase chip temperature; maintaining a minimum load

resistance of 1 kΩ is recommended.

The AD862x is designed for mounting on PC boards.

Maintaining picoampere resolution in those environments

requires a lot of care. Both the board and the amplifier’s

package have finite resistance. Voltage differences between the

input pins and other pins as well as PC board metal traces may

cause parasitic currents larger than the AD862x’s input current,

unless special precautions are taken. For proper board layout

to ensure the best result, refer to the ADI website for proper

layout seminar material. Two common methods of minimizing

parasitic leakages that should be used are guarding of the input

lines and maintaining adequate insulation resistance.

Contaminants such as solder flux on the board’s surface and the

amplifier’s package can greatly reduce the insulation resistance

between the input pin and traces with supply or signal voltages.

Both the package and the board must be kept clean and dry.

PHOTODIODE PREAMPLIFIER APPLICATION

The low input current and offset voltage levels of the AD862x,

together with its low voltage noise, make this amplifier an

excellent choice for preamplifiers used in sensitive photodiode

applications. In a typical photovoltaic preamp circuit, shown in

Figure 45, the output of the amplifier is equal to

(P)Rf

R

ID(Rf)

V

p

OUT

−

=

−

=

where:

ID = photodiode signal current (A)

P = light power incident to photodiode surface, in W

error proportional to the value of the feedback resistor. The

)

Rf(I

V

R

R

V

B

OS

D

f

E

+

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

= 1

A shunt resistance on the order of 100 MΩ is typical for a small

drops by a factor of two for every 10°C rise in temperature. In

the AD862x, both the offset voltage and drift are low, which

Figure 45 results in an SNR value of 95 dB for a signal bandwidth

of 30 kHz.

100M

Ω

C4

15pF

5pF

1.5M

Ω

OUTPUT

AD8627

PHOTODIODE

Figure 45. A Photodiode Model Showing DC Error

OUTPUT AMPLIFIER FOR DIGITAL-TO-ANALOG

CONVERTERS

Many system designers use amplifiers as buffers on the output

of amplifiers to increase the DAC’s output driving capability.

The high resolution current output DACs need high precision

amplifiers on their output as current to voltage converters (I/V).

Additionally, many DACs operate with a single supply of 5 V. In

a single-supply application, selection of a suitable op amp may

be more difficult because the output swing of the amplifier does

not usually include the negative rail, in this case AGND. This

can result in some degradation of the DAC’s specified perform-

ance unless the application does not use codes near zero. The

selected op amp needs to have very low offset voltage—for a

14-bit DAC, the DAC LSB is 300 µV with a 5 V reference—to

eliminate the need for output offset trims. Input bias current

should also be very low because the bias current multiplied by

the DAC output impedance (about 10 kΩ in some cases) adds

to the zero code error. Rail-to-rail input and output performance

is desired. For fast settling, the slew rate of the op amp should

not impede the settling time of the DAC. Output impedance of

the DAC is constant and code independent, but in order to

minimize gain errors, the input impedance of the output

amplifier should be as high as possible. The AD862x, with very

amplifier for these types of applications. A typical configuration

with a popular DAC is shown in Figure 46. In these situations,

the amplifier adds another time constant to the system, increasing

the settling time of the output. The AD862x, with 5 MHz of BW,

helps in achieving a faster effective settling time of the combined

DAC and amplifier.